Flip Chip Attachment

There is virtually nothing about flip chip attachment that is standard, due in large part to the bumps that make contact between the chip and the substrate. The bumps may consist of solder alloy, polymer, pure indium or gold alloy and range in size from 1 mil on 1 mil centers to 10 mil on 10 mil centers. They may be located on the periphery of the package or in an array or staggered array arrangement. Even the process used to deposit them on the wafer before it is cut up into individual die may vary.

Although assembling flip chips onto substrates can be achieved in different ways, its success depends on the use of versatile and flexible equipment during the assembly process. The following steps take place in a real-world application and describe the different processes of solder reflow (from the point of pick-up through reflow), inspection and rework.

1) BumpingThe flip chips in this application were bumped with lead/tin solder on FR4 epoxy fiberglass board (Figure 1 - return to this page with your BACK button) using a wafer bumping technique called Flex or Cap (FOC). Figure 2 shows a cross section of the under-bump-metallurgy (UBM) and a solder paste bump as a means of illustrating the architecture involved in this bumping process. The following basic assembly steps could also apply to bumps involving screened-on polymer, Z epoxy, UV curing epoxy, gold alloy, indium alloy or aluminum.

Figure 1:An FR4 epoxy fiberglass board was used as the basis for flip chip attachment.

Figure 2: A Cross-section of a sputtered UBM and solder paste illustrates the principle of the Flex on Cap (FOC) wafer bumping approach.

2) Presentation and Pick-upAlthough bumped die can be presented to the assembly equipment in various forms, one common method is in a waffle pack, with the bumps oriented downward and touching the bottom. However, in some cases the bumps cannot be touched (e.g., because of their softness) and must be presented bumps up, thus requiring the use of an inverter.

Pick-up from the waffle pack, alignment of the chip's solder bumps with the substrate's bond pads and placement are all accomplished using specialized equipment.

Operation of such equipment is relatively simple. In the case of the pick-up cycle, the chip must simply be targeted so that the operator can align it to the system´s pick-up tool (Figure 3). The system´s pick-up cycle is then initiated and the machine automatically picks up the flip chip. For chips presented with their bumps up, some bonders are outfitted with an optional inverter that picks up the chip, turns it over and passes it off to the system's placement tool.

Figure 3: The Chip must be manually aligned to the pick-up tool before the system's picl-up cycle can begin.

3) FluxingTo begin the flux operation, a manual doctor blade spreads a thin film of flux evenly on the flat surface of the flux tray on top of the fluxing station/pedestal (Figure 4). If the chip is dipped into too much flux, it will adhere to the flux in the tray and be pulled off of the vacuum tool (pick-up head). For some epoxy applications, it is practical to spread a thin film of epoxy in the tray and then dip the flip chip in it, so that only the chip's contact areas are coated with the epoxy. To dip the chip into the film of flux or epoxy, the operator simply aligns the chip with the fluxing tray and initiates the dipping cycle, which is then carried out by the machine. The sticky nature of the flux causes the chip to stay in position once it has been placed on the target site.

Figure 4: When spreading flux on the flux tray, it is important to keep the layer thin and even so that the chip will not adhere to the flux and be pulled off of the vacuum tool.

4) AlignmentThe cube beam splitter used to align the FOC-bumped chips presents real-time views of the chip´s bumps and substrate bond pads superimposed on each other.

To prepare for chip placement, the operator aligns the bumps over the bond pads on the substrate, the latter having been previously placed on the system´s workstage to the right of the pick-up head. The operator then moves the substrate into position under the pick-up head, at which point the system´s cube beam splitter extends out to a position between the pick-up head and the substrate and presents a view of the chip's bumps and substrate bond pads (Figure 5). Adequate lighting of both the flip chip bumps and substrate is attained with separate, adjustable illuminators - one for the bumps and one for the bond pads.

Figure 5: A cube beam splitter view of chip bumps and bond pads help insure perfect alignment between the die and the substrate.

5) PlacementDuring placement, the chip is automatically placed under bond load before the pick-up tool retracts to its upper position. The bond load should be just enough to seat the chip properly on the substrate. To ensure good reflow, each of the chip bumps must be touching a bond pad on the substrate. When flux cannot be used to hold the chip in place during reflow, a machine with elevated stage temperatures and higher bond loads should be used (Figure 6). When solder is used, it is only necessary to apply some amount of bond load and then retract. However, with epoxy and adhesive applications, the chip needs to be held at a load of up to 10 kg while curing takes place. With microwave applications that involve a smaller number of contacts, the flip chip can be thermosonically attached using the bonder´s ultrasonic scrub option with elevated heat. Variations on the ultrasonic scrubbing action incorporate vertical movement to achieve partial attachment at a lower temperature.

Figure 6: The higher elevated temperature of machines (such as this Model 410 precision chip bonder) softens the solder, so that the chip will be tacked firmly in place when the bond load is applied.

6) ReflowReflow usually occurs on the same machine that places the flip chip. The operator moves the die from the placement position to the reflow position, then depresses a foot petal which activates a single stationary shrouded hot gas heating nozzle (Figure 7). Upon reflow, a timer signals the end of the cycle and the foot peddle is released, shutting off the gas.

With a multichip device reflow is performed in a controlled atmosphere oven. The flow-through design oven2 used to reflow some of the flip-chipped substrates in this application (Figure 8) combines bottom-up conduction and top-down convection heating with precise calibration of the temperature and purity of the atmosphere within each of its four heat zones and two liquid cool zones. The temperature of the bottom conduction platen and upper convection platen in each zone can be individually calibrated and controlled on a repeatable basis to a setpoint, and the inert atmosphere on each zone can be purged to 10 ppm oxygen.

When reflowing miniature designs, hot gas cannot be used because the velocity from the bonder's hot gas nozzles is too high (even after throttling down) for the lightweight small chips. Instead, either a Xenon lamp system (with the visible light being delivered through a fiber optic bundle) or an infrared laser system is used on the bonder to reflow the gold contacts. There are certain laser diode applications, for example, that call for accuracies within 1 µm, which is beyond the capability of available flip chip bonders. In such situations, etching, lithography, and vacuum deposition technologies are used. The bonder is then used to place the chip and the self alignment phenomenon maintains the 1 µm precision achieved in the bumping process.

When flip chips use polymer bumping instead of solder, the bonder holds the chip on the substrate using some amount of bond load, and then heats to achieve a snap cure or full cure.

Figure 7: The reflow cycle on a manual flip chip placement system is initiated with a foot pedal to activate the shrouded hot gas heating nozzle.

Figure 8: The substrate enters the conduction/convection reflow oven as the first step in an automatic reflow operation.

7) Inspection and ReworkFlaws (such as cracks or voids) caused by the design and/or attachment technique must be detected and eliminated before proceeding. Ultrasonic frequencies ranging from 5 to 200 MHz can nondestructively characterize the homogeneity and bonding quality of materials. At these frequencies, ultrasound is extremely sensitive to elastic properties and will not transmit through air. It is an ideal tool to quantify the physical characteristics or materials, locate internal defects and measure the bond quality between two or more surfaces. Figures 9a and 9b show an examination of the solder reflow of two of the flip chip interconnections made during the course of the assembly described here.

Should the inspection indicate that a flip chip needs to be replaced on the substrate, the rework can than be accomplished with spot heating via a flip chip bonder. Using an oven is not an option, since it would reflow everything on the substrate. After aligning the system´s pick-up head over the defective flip chip, the two hot gas nozzles are lowered over the rework site (Figure 10) just as they would be for reflow after initial placement. During rework, however, the temperature of the solder is elevated beyond reflow. The nozzles then retract and the defective chip is lifted off of the substrate. Finally, the site on the substrate is cleaned in preparation for pick-up, alignment, placement, and attachment of the replacement chip.

Figure 9: In Figure 9a, the bonded bumps (at the board level) appear as dark spots in a rectangular pattern at the edges of the metal traces on the substrate. There is some variation in the size of the bonds in this sample and bridging is evident in several areas (arrows). Figure 9b shows the bonds (at board level) as dark spots in a rectangular pattern at the edges of the metal traces on the substrate. The size of the bonds is shown to be fairly uniform.

Figure 10: The bonder's hot gas nozzles are lowered onto the rework site in preparation for the reflow and removal of the defective chip.